With the incidence of over five million new HIV infections each year the development of a HIV vaccine has become a global priority. An effective vaccine against HIV infection should induce strong cellular responses as well as antibody responses. Although there are several candidate vaccines that are currently under clinical trials there is still a critical need to design efficacious HIV vaccines. The objective of this proposal is to enhance the efficacy of HIV vaccines by transiently blocking regulatory T cells (Tregs) during immunization. Tregs, also known as suppressor T cells, serve as the "brakes" of the immune system. They dampen immune responses and limit the magnitude of immune responses. Recent studies have shown that neutralization of Treg activity leads to enhanced immune responses. Tregs express on their surface, the glucocorticoid-induced tumor necrosis factor receptor (GITR). Engagement of GITR on the Tregs by its ligand, GITRL (which is expressed on dendritic cells, macrophages and B cells) neutralizes Treg function. Recent studies also show that engagement of GITR not only turns off Tregs but also co stimulates conventional CD4+ T cells. The central hypothesis of this grant proposal is that neutralizing Treg function via GITR signaling will lead to enhancement of vaccine-induced T and B cell responses to HIV envelope antigen. We will test this hypothesis by immunizing mice with DNA vaccines that dually encode both, HIV envelope antigen and GITRL. We will accomplish the goal of this proposal by pursuing the following two specific aims. Specific Aim 1: To determine the extent to which co-expression GITRL enhances T cell responses to HIV envelope glycoprotein. Specific Aim 2: To determine the extent to which co-expression GITRL enhances B cell responses to HIV envelope glycoprotein. Taken together, these studies will determine the extent to which transiently blocking Tregs during immunization, enhances T and B cell responses to HIV envelope. It is our expectation, based upon strong preliminary data, that the proposed experiments will be successful.